The purpose of this project is to study the function of the aortic and mitral valves in dogs with acute heart failure by using high-speed (500 frames per second) cineradiography. The cardiac valves are made visible by appying a viscous contrast agent to their distal surface and in some animals by inserting small, 5 mg., tantalum screws on the leaflets. Four major changes in cardiac function are produced: Increased heart rate through pacing, increased preload through infusion of dextran, decreased inotropy by administration of high doses of halothane, and increased afterload through infusion of angiotensin II. We have found that over a wide range of heart rates (60 to 200 beats per minute) the average duration of opening is 29.0 0.55 milliseconds ( S.E.) and that this time to maximum opening is not dependent on heart rate r equals 0.42). However the average duration of closing was 142 2.1 milliseconds and this interval was dependent on heart rate (r equals 0.82). In order to determine the relationship between stroke volume and aortic valve function a new noninvasive means for estimatining changes in stroke volume was investigated. It was found that the amplitude of the intratracheal pressure oscillations produced by cardiac contraction (the pnuemocardiogram) was linearly related to changes in stroke volume (r equals 0.90). Therefore the pneumocardiogram can not only be used in the investigation of valve function but it may also be of great value in determined changes in stroke volume during surgery on humans.